Note that this PR was made based on the top of https://github.com/apache/spark/pull/20151. So, it almost leaves the main codes intact.

This PR proposes to add a script for the preparation of automatic PySpark coverage generation. Now, it's difficult to check the actual coverage in case of PySpark. With this script, it allows to run tests by the way we did via `run-tests` script before. The usage is exactly the same with `run-tests` script as this basically wraps it.

This script and PR alone should also be useful. I was asked about how to run this before, and seems some reviewers (including me) need this. It would be also useful to run it manually.

It usually requires a small diff in normal Python projects but PySpark cases are a bit different because apparently we are unable to track the coverage after it's forked. So, here, I made a custom worker that forces the coverage, based on the top of https://github.com/apache/spark/pull/20151.

I made a simple demo. Please take a look - https://spark-test.github.io/pyspark-coverage-site.

[This scope](https://github.com/apache/spark/blob/04e44b37cc04f62fbf9e08c7076349e0a4d12ea8/python/pyspark/daemon.py#L148-L169) in `daemon.py` is not in the coverage results as basically I am producing the coverage results in `worker.py` separately and then merging it. I believe it's not a big deal.

In a followup, I might have a site that has a single up-to-date PySpark coverage from the master branch as the fallback / default, or have a site that has multiple PySpark coverages and the site link will be left to each pull request.

## How was this patch tested?

Manually tested. Usage is the same with the existing Python test script - `./python/run-tests`. For example,